All electronic imaging products must achieve and maintain a level of picture quality which is acceptable to the user. One important aspect of picture quality is the signal-to-noise ratio (SNR) which is a measure of video signal amplitude versus any unwanted noise signal amplitude. The human vision system is sensitive to most electronic noise which can be introduced into the video signal. This noise most often has a repetitive nature which is sometimes synchronized with the video information being displayed and at other times can be non-synchronous which causes it to move or roll with respect to the video. Therefore, a large SNR of 40 dB or greater is normally required for electronic imaging products.
One source of electronic noise is the power supply which provides the appropriate voltage levels required by the electronic components. The generation of multiple, different voltage levels from one common input voltage has been accomplished in numerous ways including switching transformer DC to DC converters. The draw back of these standard solutions is the unacceptable voltage ripple on the output voltage levels which are synchronized to the switching frequency that has been selected to optimize the voltage conversion. This switching frequency is usually an independently generated clocking signal which controls the power conversion operation and typically ranges from 100 Khz to 500 Khz. Since this switching frequency source is independent it will tend to beat with any other independent clocking frequency operating in the circuitry. The residual noise component due to the power converter clocking frequency can also be riding on top of the output voltages generated by such a power converter. In the prior art, additional filter components are added to reduce the remaining switching noise on the output voltage levels. These methods do help reduce the noise affects, but in video electronics it is almost impossible to eliminate this noise completely using additional filter components. This is due to many aspects such as human vision integrating the video information on a display which can distinguish very low levels of rolling, fixed pattern noise. Video electronics must maintain good signal to noise ratio and dynamic range to provide an optimum imaging system. This usually requires a large signal gain for low light conditions which tends to also magnify the noise sources present such as power supply ripple.
One prior art switching transformer DC to DC converter uses a single ended, switching regulator controller to form a flyback switching transfer circuit. This circuit is synchronized to the horizontal line rate and uses pulse width modulation to control the amount of time that current is conducted through the primary winding of a transformer. Output pulse modulation is accomplished by comparison of the positive saw tooth waveform to either of two control signals at the error amplifier inputs. This allows the error amplifiers a means to adjust the output pulse width from a maximum percent on-time down to zero percent. Modulation of the output pulse width controls the amount of current passing through the transformer's primary which determines the transformer's internal magnetic flux intensity and therefore controls the amplitude of the transformer's output voltage on its secondary windings.
The drawback of this approach is that it relies upon pulse width modulation of the primary current to control the transformer's output. This allows for on to off primary current transitions to occur during the active video display time and any resulting supply ripple can be amplified and show up as noise on the video display. This on to off transition point may also vary with respect to time as the pulse width is varied to compensate for a changing load requirement on the transformer's secondary. This could result in a moving, fixed pattern noise component on the video which is easily discernible to the observer.
Another drawback of the above described circuit is the unsymmetric nature of the flyback transformer configuration. This is because the primary winding actually conducts current in only one portion of the duty cycle of the pulse width modulated control signal. The remaining portion of the duty cycle that does not conduct current through the primary and control switch must rely upon the energy stored within the primary inductance to provide enough flux linkage to maintain the proper secondary voltage levels. This tends to lead to an unsymmetrical secondary voltage modulation that can vary in duration depending on the pulse width modulation of the control signal.